doubt its legit, they never come up with the story, then build it. They look at what story they can make, and what technology they have to pull stuff off. One of the original writers retired and said this in an interview about what happened during his time with Valve.

A bit of a related thing. I once read the Halo3 story when I realised it would never hit PC. I'd played 1 and 2, I would have played 3 if it came to PC

As for HL3 I would say that there is always a chance. There is zero chance of HL2:Episode 3. Other franchises have gone dark for long periods of time. Ultimately there is money on the table since the Half Life franchise is so popular. At some point someone will go back to that well.